An aspect of this invention relates to low noise amplifiers (LNAs).
Today, storage devices, such as hard disk drives, are operating at close to Gigabit/s data rates, and soon will reach 2 Gigabit/s data rates. At these data rates, signal bandwidth may approach Gigahertz levels; similar to wireless applications. As such, it is now important to utilize low noise amplifier circuitry having matched input impedance to achieve a flat frequency response when interfacing the amplifier circuitry to a storage device read head through a transmission line. This is similar to wireless applications, where a matched impedance low noise amplifier is generally needed to interface to the antenna and the corresponding transmission line attached to it.
However, unlike wireless applications, storage devices are more broadband in nature. In fact, most storage devices such as a hard disk drives produce signals from about DC to the Nyquist frequency. In contrast, wireless devices typically operate over a narrow frequency band (at most up to a few tens of Megahertz) with the signals centered around the carrier frequency. For the purpose of matching the input impedance of a conventional RF wireless amplifier, the impedance only needs to be matched at the carrier frequency. Input impedance matching of conventional wireless amplifiers can simply be achieved using resonance tuning with inductive and capacitive components as is well known to those skilled in the art.
Tuning schemes do not work on ultra broadband signals such as that found in a disk drive device and the emerging ultra wideband wireless devices that are being debated in the industry. For these ultra broadband and ultra wideband devices to work better, a very low noise amplifier is needed to process the weak signals encountered by the read head or the antenna. Since a transmission line is normally used to couple the read head or the antenna to the low noise amplifier, input impedance matching is needed.
Conventional very low noise amplifiers and other wideband amplifiers generally include a passive resistor to impedance match to the transmission line. However, using a passive resistor may increase power loss and the input noise figure (sometimes referred to as input referred noise voltage) due to the fundamental noise generated by the resistor. Circuit textbooks universally teach that any resistive components, real resistors or synthesized resistors, generate wideband thermal noise. The value of the fundamental noise is widely known to be {square root over (4 kTR)} for ideal resistors and somewhat higher for synthesized resistors.
A low noise amplifier (LNA) for amplifying an input signal communicated over a transmission line having an impedance. The LNA includes a current sensing amplifier having an input to connect to the transmission line. The current sensing amplifier has an input impedance that matches the transmission line impedance. The current sensing amplifier amplifies the input signal to generate a first output signal. A voltage sensing amplifier receives the input signal and generates a second output signal. A combiner combines the first output signal and the second output signal to generate an LNA output signal.